1. Field of the Invention
The present invention relates to a method for producing silicon single crystal, which can be used as a material for semiconductor devices, solar cells and the like.
2. Background of the Invention
Single-crystal material, in particular single-crystal silicon in large volume, is an essential material for the development of highly effective solar batteries (solar cells). There are primarily two processes for forming single-crystal silicon. A single-crystal silicon rod obtained by the Float Zone (FZ) method, is recognized to have the highest quality among current methods. The Czochralski (CZ) method, an alternative to the FZ method for providing single-crystal silicon, cannot eliminate the defects caused by dissolved oxygen due to the use of a silica (quartz) crucible. FIG. 2 shows a manufacturing method for a single-crystal silicon rod according to the FZ method.
The conventional manufacturing method for a single-crystal silicon rod in accordance with the FZ method uses as a starting material, a polycrystalline silicon rod manufactured by the Siemens process. The Siemens process is described in Ger. Offen 1,102,117. A polycrystalline silicon rod (50) is partially melted by inserting the material into an induction coil (40a) to form a single-crystal silicon rod (51) which is further inserted into a second induction coil (40b) to provide the product of a single-crystal silicon rod (52). Two FZ melting steps are required to fully exhaust gas contained in the material (50) as instructed in manufacturing manuals published by the developers of the FZ method.
The conventional method provides a single-crystal silicon rod of a long lifetime, high specific resistance and the like, each essential for the solar cells, but at a high cost and price. The economics of production by the conventional method hinders provision of single-crystal silicon in a large volume for solar cells.
The reasons for the high cost to produce the single-crystal silicon by the FZ method are mainly as follows:
(1) There is a high cost to produce the starting material polycrystalline silicon rod by the Siemens process.
(2) There is high loss of the polycrystalline silicon rod manufactured by the Siemens process in preparing the rod for FZ melting. Due to irregularities in the outer diameter of the rod manufactured by the Siemens process, the outer surface of the rod must be shaped in order to fit between the induction coils for FZ melting. This introduces waste of polycrystalline silicon.
(3) Two FZ melting process steps must be performed to obtain high quality single-crystal silicon rods. A single FZ melting is insufficient to obtain high quality single-crystal silicon.
The reasons for the high cost associated with producing polycrystalline silicon by the Siemens process is as follows. This process is conducted basically in batch production and is thereby low in production efficiency. The Siemens process has the additional drawback in that the surface area for deposition of silicon is smaller with respect to the furnace capacity. Further, the furnace surface dissipates heat rapidly, thereby leading to a high cost of production.
In recent years, the fluidized-bed granulation process, as an alternative to the Siemens process has been given attention as a manufacturing method for polycrystalline silicon. The applicant has developed this process (Japanese Patent Applications Nos. 324010/1988, 100929/1989, 100930/1990).
The fluidized-bed granulation process continuously grows polycrystalline silicon granules in a fluidized-bed reactor (FBR) and extracts them therefrom, so that the process is substantially high in production efficiency in comparison with the Siemens process of batch production. Further, the ratio of the surface area for deposition of silicon with respect to the capacity of a FBR is especially larger, thereby providing definite advantages in productivity, efficiency of power consumption and the like, and a notably lower cost to produce polycrystalline silicon in comparison with the Siemens process. While fluidized-bed granulation efficiently produces polycrystalline silicon granules, the granules are inappropriate for direct crystallization by the FZ method due to low quality as well as small size.
Despite the existing methods, no one has provided an economical process for producing polycrystalline silicon rods of high quality from the cheap granules.